Horizon sensors for sensing the infra-red radiation from the earth's atmosphere and determining an arbitrary "edge" of such atmosphere are well known. One type utilizes a circular array of infra-red detectors nominally arranged around the earth's image and deviation of the earth's horizon from the center of the circular array is related to a change in pitch or roll from the null angle (zero pitch and zero roll). Pitch and roll of course relate to what is known as the attitude of the spacecraft; roll is displacement about the longitudinal axis of the spacecraft and pitch about its lateral axis (perpendicular to the longitudinal axis).
While such technique is accurate and reliable, the versatility and cost may be limited. It is desired to provide at low cost a horizon sensor which will be versatile for any spacecraft mission profile, be it low earth orbit (LEO), geosynchronous earth orbit (GEO), or transition earth orbit (TEO).